1. Field of the Invention
The present invention relates to a film forming apparatus and method for forming functional deposited films for photovoltaic elements or various kinds of sensors. More particularly, the invention relates to an improved film forming apparatus and method in which the maintenance time for continuous film formation such as a roll-to-roll film formation is shortened to enhance the working efficiency of the apparatus.
2. Related Background Art
A variety of semiconductor devices or electronic devices including photovoltaic elements or various kinds of sensors are provided with one or more functional deposited films on a substrate. In a manufacturing process of such devices, it is desired to form the deposited films as having certain level of characteristics continuously and efficiently to mass produce the products of superior characteristics.
For example, in a photovoltaic element such as a solar cell of the structure in which a plurality of semiconductor layers (i-layer, n-layer, p-layer) are laminated, various examinations for the stable film forming process have been made to enhance its function.
In recent years, a power generation system with a solar battery using the sunlight has drawn attention as a clean power generation system which can cope with increased demands for electric power in the future, without causing environmental destruction, since it does not bring about problems of radioactive pollution or global warming, uses a less maldistributed energy source, and further can accomplish a relatively high efficiency of power generation without needing complex and large installations. Various activities of research and development have been made for practical use of such a battery.
To establish the power generation system using the solar battery as meeting the demands for electric power, it is fundamentally required that the solar battery has a high enough photoelectric conversion efficiency, stable characteristics, and is capable of mass production.
In this respect, a solar battery which can be fabricated, using a source gas such as silane, in a gaseous body, which is easily available, by depositing a semiconductor film of e.g. amorphous silicon (hereinafter abbreviated as "a-Si") on a relatively inexpensive substrate made of glass or metal has been noted such a battery is suited for mass production, with the possibility of lower production costs, as compared with the solar battery fabricated using a single crystal silicon. Various proposals have been made for the constitution of the basic layers and the manufacturing methods thereof.
Although a-Si deposited film is formed on a band-like substrate by chemical vapor deposition (CVD) which typically occurs from the gas phase under reduced pressures, or sputtering, a plasma CVD method making use of glow discharge plasma is widely utilized because the characteristics of the deposited film, are superior and can be mass produced.
Recently, a plasma process making use of microwave has been also noted. The microwave, which is short in frequency band, can have a higher energy density than when using RF, and thus is suited for generating and sustaining plasma efficiently.
For example, in U.S. Pat. Nos. 4,517,223 and 4,504,518, a method of depositing a thin film on a substrate of small area within microwave glow discharge plasma under low pressures has been disclosed. With this method, since the film formation can be made via a process under low pressures, high quality deposited films can be produced by preventing polymerization of active species which may cause degraded film characteristics. In forming an Si film, the film forming speed can be remarkably increased while generation of polymers such as polysilane in plasma is suppressed.
However, in microwave plasma, though much higher film forming speed may be typically expected, microwave applicator means making use of a microwave generator, an isolator, a waveguide, and an arsela window is needed to introduce microwave into a film forming chamber, resulting in higher costs than the conventional RF methods. Accordingly, for example, in the formation of a-Si film in manufacturing the a-Si solar cell, the microwave is used for fabrication of a photovoltaic layer (i-type a-Si layer) having large film thickness for which high throughput is required, while the RF method is used for making other layers, i.e., n-type a-Si layer and p-type a-Si layer. A so-called hybrid method has thus been proposed.
On the other hand, from the viewpoint of a film formation process, and in consideration of mass production of final devices, a continuous plasma CVD system which adopts a roll-to-roll (Roll to Roll) type substrate which is wound like a roll has been disclosed in U.S. Pat. No. 4,400,409.
With this apparatus, a plurality of glow discharge regions are provided, a flexible substrate having a desired width and a sufficient length is laid along a path extending through the glow discharge regions, through which the substrate is passed successively. A semiconductor layer of the required conduction type is deposited in the glow discharge regions, while the substrate is conveyed continuously in a longitudinal direction, to allow for the continuous formation of elements having semiconductor junctions.
Note that in U.S. Pat. No. 4,400,409, a gas gate was used to prevent dopant gas for use in forming each semiconductor layer from diffusing and mixing into other glow discharge regions. Specifically, the glow discharge regions are separated from one another by a slit-like separation passage, and the separation passage is provided with means for forming the flow of scavenging gas such as Ar or H.sub.2. In this respect, it can be said that the roll-to-roll type is suitable for mass production of semiconductor devices of the structure in which various functional films are laminated.
In addition, a continuous plasma CVD system of roll-to-roll type for forming a large area and a-Si deposited film was disclosed in U.S. Pat. No. 4,485,125.
For a plasma process using microwave, a deposited film forming method and apparatus of roll-to-roll type using a microwave plasma CVD system was disclosed, for example, in Japanese Laid-Open Patent Application No. 3-30419.
A typical plasma CVD system of roll-to-roll type will be described below with reference to the drawings.
FIG. 1 is a cross-sectional view showing the constitution of the plasma CVD system of roll-to-roll type, and FIG. 2 is a cross-sectional view showing the constitution of a vacuum vessel (chamber) and a film forming chamber which are contained in the apparatus. In FIG. 1, 500 to 504 are vacuum vessels (chambers), 505 to 507 are film forming chambers, 508 to 510 are discharge electrodes, 511 to 513 are glow discharge spaces, 514 to 516 are rf oscillators, 517 to 519 are substrate heaters, 520 to 522 are gas heaters, 523 to 525 are source gas inlet ports, 526 is a magnet roller, 527 to 530 are gas gates, 531 to 533 are exhaust pumps, 534 to 535 are pressure gauges, 537 to 540 are gate gas inlet ports, 541 is a delivery bobbin, 542 is a winding bobbin, and 543 to 545 are gas gate exhaust ports.
FIG. 2 is a cross-sectional view of the apparatus (one vacuum vessel) as seen from the side. The prismatic vacuum vessels 500 to 504 are arranged on a straight line as viewed from the upper face, or like a catenary as viewed from the lateral side. The substrate for forming the film thereon is a band-like substrate 100 having a desired width and a sufficient length.
Provided inside the vacuum vessels 501 to 503 are film forming chambers 505 to 507, respectively, in which desired semiconductor layers are formed on the band-like substrate 100 within the glow discharge spaces 511 to 513 which are enclosed by the band-like substrate 100, the discharge electrodes 508 to 510, and the film forming chambers 505 to 507.
In FIG. 1, a vacuum vessel 500 at the left end as shown contains the delivery bobbin 541 for the band-like substrate 100, and a vacuum vessel 503 at the right end as shown contains the winding bobbin 542.
The band-like substrate 100 extending lengthwise as a band is made of a material having electrical conductivity, flexibility and magnetism such as stainless steel, is delivered from the delivery bobbin 541 to pass through the vacuum vessels 501 to 503 and the film forming chambers 505 to 507, in succession, and wound around the winding bobbin 542.
On a conveyance passageway of the band-like substrate 100, there are disposed a plurality of magnet rollers 526 at appropriate locations thereof, which are magnetized and rotatable, to support the band-like substrate by magnetic suction to retain a predetermined conveyance passageway.
In FIG. 1, the film forming chambers 505 to 507 are connected to exhaust pumps 531 to 533 having an exhaust speed regulating function, to keep the inside of film forming chambers at desired pressures by measuring the pressure by means of the pressure gauges 534 to 536, and controlling the exhaust speed of the exhaust pumps 531 to 533 by means of a pressure control device (not shown).
A plurality of different kinds of source gases are mixed into desired constituents by means of a gas mixer (not shown), and this mixed gas is fed through source gas inlet ports 523 to 525 into the film forming chambers. Also, the vacuum vessels are connected via the gas gates 527 to 530, respectively, which have both functions of preventing mutual diffusion of source gases of adjacent vacuum vessels by isolating them, and passing the band-like substrate 100 therethrough.
A gas isolating function can be fulfilled by connecting adjacent vacuum vessels via a slit-like separation passageway, and flowing a separation gas (gate gas) from the upper and lower faces of the gas gate to collide with the source gas to shorten the diffusion length of the source gas.
Examples of the gate gas include H.sub.2, He and Ar. The exhaust pipes connected to the film forming chambers 505 to 507 are provided with the gate gas exhaust ports 543 to 545, respectively, whereby the source gases or decomposed gases which flow out of the glow discharge spaces 511 to 513 are exhausted from the gate gas exhaust ports 543 to 545, together with the gate gas, to prevent the gate gas and the mixed gases from adjacent vacuum vessels from entering the glow discharge spaces.
The above-described constitution of the apparatus of a double chamber structure having vacuum vessels and film forming chambers, with a gas gate provided between adjacent vacuum vessels, is an important technique for the plasma roll-to-roll type CVD system.
Even if the band-like substrate 100 is moved into a next vacuum vessel, the source gas within the vacuum vessel is not transferred. Further, even if there is any pressure difference within each vacuum vessel, mutual diffusion or mixture of source gases between adjacent vacuum vessels can be suppressed to A minimum, so that the semiconductor layers of desired conduction type having excellent characteristics can be deposited in succession on the band-like substrate 100.
Referring now to FIG. 2, the internal structure of vacuum vessels 501 to 503 containing the film forming chambers 505 to 507 of FIG. 1 will be described below.
In FIG. 2, 700 is a film forming chamber, 701 is a feeder board, 702 is a shield, 703 is an rf introducing flange, 704 is a gas introducing flange, 705 is a gas introducing tube, 706 is a film forming chamber stay, 100 is a band-like substrate, 101 is a vacuum vessel, 102 is a ceiling plate, 104 is a discharge electrode, 105 is a guard electrode, 110 to 112 are insulators, 113 is a substrate heater, 114 is a gas heater, 115 is a heater supporting stay, and 118 is a glow discharge space.
In FIG. 2, RF power from the rf oscillator (not shown) is supplied via the rf introducing flange 703, and the feeder board 701, to the discharge electrode 104. Also, the source gas is supplied via the gas introducing flange 704 into the film forming chamber 700.
In FIG. 2, a space enclosed by the film forming chamber 700, the band-like substrate 100, the ceiling plate 102 and the discharge electrode 104 is the glow discharge space 118.
By decomposing the source gas introduced into the glow discharge space 118 with RF power applied to the discharge electrode 104, the semiconductor deposited film of desired conduction type can be formed on the band-like substrate 100. The ceiling plate 102 is attached to the film forming chamber 700, to form an upper lid of the film forming chamber 700, along with the band-like substrate 100.
Also, on the back face side of the band-like substrate 100, the heater 113 is provided to heat the band-like substrate 100 to a proper substrate temperature. The source gas is heated by the gas heater 114, and fed into the glow discharge space 118. Also, the gas heater 114 heats the film forming chamber 700, the discharge electrode 104 and the guard electrode 105.
Heating of these has the effect of preventing the powder of polysilane produced by decomposition of the source gas from depositing on the wall face of film forming chamber 700 as well as the surface of discharge electrode 104.
In order to form the film to be deposited on the band-like substrate 100, with good controllability and reproducibility, it is requisite to introduce the RF power and the film forming gas into the film forming chamber 700 without leakage.
For example, to keep the RF power from leaking from the shield 702, it is desired that the shield 702 is rigidly attached to a bottom face of vacuum vessel 101 without clearance, and that the feeder board 701 is made of copper having a high electrical conductivity.
However, because copper has also a high thermal conductivity, the feeder board 701 will be heated by thermal conduction from the discharge electrode 104 heated by plasma of the glow discharge space 118 and the gas heater 114.
It is necessary to provide such a design that the feeder board 701 having caused thermal expansion does not make contact with the shield 702 by deformation.
Further, it is necessary to provide such a design that the source gas is introduced into the film forming chamber 700, and may not leak into the vacuum vessel 101 except for the film forming chamber 700. From the above reasons, the use of a bellows-like flexible mechanism for the feeder board 701, the shield 702, and the gas introducing tube 705 may be considered. However, mechanism may possibly cause breakage due to changes with the lapse of time. In an apparatus which will operate long term as a production machine, because it is important to have reproducibility and reliability, the feeder board 701, the shield 702 and the gas introducing tube 705 are desirably in the form of a durable rod.
The operation of the plasma CVD system of roll-to-roll type will be schematically described in the following. In FIG. 1, if the plasma CVD system is activated, the band-like substrate 100 delivered from the delivery bobbin 541 is continuously conveyed in a longitudinal direction thereof at a constant rate, passed through the film forming chambers 505 to 507 to form desired semiconductor layers in succession on the band-like substrate 100 within the glow discharge spaces 511 to 513, and then wound around the winding bobbin 542.
Finally, a plurality of sorts of semiconductor layers are laid down on the band-like substrate 100 to continuously form desired semiconductor junction devices. As a result, the semiconductor junction devices of large area can be mass-produced.
The apparatus as shown in FIG. 1 is a plasma CVD system of roll-to-roll type to form a photovoltaic element having one pin structure, i.e., a single cell, if applied to the manufacture of photovoltaic element. However, this apparatus is able to form a so-called triple cell having a pin-pin-pin structure with enhanced photoelectric conversion efficiency, if more film forming chambers are connected.
Generally, the whole size of the apparatus may be different depending on the production throughput, but the apparatus having the capability of producing photoelectric elements of triple type which generate about 10 MW of optical power for one year has approximately 20 film forming chambers, with the overall length of apparatus being about 40 m in a longitudinal direction.
In view of an example of the film forming process for a photovoltaic element such as a solar cell, as above described, fabrication of devices having multiple functional films can be applied as a mass production method having a greater throughput by reasonably combining a continuous film forming system such as roll-to-roll production as above described with various film forming processes. However, in such a mass production system which is considered to be ideal, the following problems arose.
Active species, precursors for forming the deposited film, may deposit on some regions of the film forming chamber other than the substrate of interest in the form of powder or film.
Such film deposited on some regions other than the substrate may be exfoliated from the bottom, beyond a certain limit of thickness. Some exfoliated film pieces may stick to the substrate, yielding defective portions on the deposited film. To prevent such a situation, cleaning the film forming chamber every time a certain number of film formations is reached, or the total time of film formation is exceeded. However, it takes considerable time to clean away the powder or film. For example, if using a file or brush take off the film, more time and labor was required, and the small portion not easily accessible was difficult to clean. Also, in the case of powder, there was a risk of causing a fire.
Accordingly, a cleaning method in which only an easily detachable portion within the film forming chamber is removed, and reused by etching or blasting for regeneration.
Even with this method, it took much time to perform mounting or dismounting of individual parts, assembling, and the etching process, resulting in reduced availability of the apparatus.
A further problem, when using the roll-to-roll type apparatus, is handling of the band-like substrate at the time of maintenance.
When the film formation for the band-like substrate one wind of bobbin is completed, one does not fully take out the band-like substrate from the apparatus. Since it is difficult to pass a new band-like substrate through the film forming chambers and the gas gates, due to narrowness of a gap of gas gates (normally from 1 mm to 10 mm), it is common practice that the leading end of a new band-like substrate is bonded to, by adhesion or welding means, and pulled by the trailing end of the previously film formed band-like substrate in order to pass throughout the apparatus.
That is, the band-like substrate always exists in the film forming chamber, thereby hampering the maintenance operation for the film forming chamber. Sometimes a desired component can not be taken out due to presence of the band-like substrate.
FIG. 3 is a typical view representing such a situation. In FIG. 3, 301 is a vacuum chamber, 302 is a gas gate, 303 is a band-like substrate, 304 is an upper lid, and 305 is a film forming chamber.
As will be clear from the figure, the parts constituting the film forming chamber, can not be easily taken out, because the band-like substrate 303 is an obstacle. Also, the band-like substrate 703 is difficult to clean on the bottom side. Namely, with a batch-type film forming apparatus, the maintenance operation can be performed after taking out the substrate, while with a roll-to-roll type, the maintenance was difficult, because the substrate remains in the form of a lengthwise continuous body.
Also, with the film forming apparatus and process making use of the above-described band-like substrate, problems such as discharge leakage, deformation of band-like substrate, difficulty in conveying the band-like substrate, rupture, or short-circuit of rf introducing portion, may occur as a result of repeated heating and cooling of the vacuum vessels and film forming chambers by operation and stop of the apparatus.